Characteristic clustering scales, Δ (see text), plotted versus cluster Age. The upper panel shows Δ for PMS members of mass above the approximate completeness limit defined in Sect. 3.1.2. The relatively more concentrated NGC 2362 is marked in red. In the lower panel, the ratio between the Δ values of the lower mass (1–2 M_⊙) and higher mass (2–3.5 M_⊙) PMS members is shown. Representative points for NGC 2264 and NGC 1893 are shown for comparison (crossed circle and star, respectively. See Sect. 2.3). Vertical error bars are calculated with the assumption that the member numbers have an absolute error equal to their square root. A linear squares fit with exclusion of NGC 3293 and Trumpler 18 (red symbols), and a correlation coefficient of 0.7, is plotted as a straight green line.